Owing to the increase in the length and the flexibility of rotor blades in wind turbines, the control units thereof are increasingly intended to reduce loads and oscillations of the wind turbine which occur during operation. Loads of the rotor blades are primarily caused by aerodynamic effects. However, a direct measurement of the aerodynamic loads is not possible. Since the blade bending actions and the blade loads are closely related to each other, information relating to the blade bending actions constitutes good initial information for a control system of a wind turbine which such information can reduce undesirable, cyclical blade loads which can be attributed, for example, to incorrect orientation of the rotor shaft with respect to the wind direction or to vertical or horizontal wind shears.
In document NREL/TP-500-39253, published in January 2006, a method for measuring a rotor blade bending action is described according to which an infrared camera arranged in the vicinity of the blade root having an infrared radiation source and reflector strips secured to the rotor blade are used. The reflector strips which are illuminated by means of the infrared radiation source reflect the infrared radiation back to the camera so that the blade bending can be calculated by evaluating the image taken by the camera.
WO 2010/054661 A2 discloses a method for monitoring the loading of rotor blades of a wind turbine, which comprises a gondola and a rotor hub which is rotatably supported thereon by means of a rotor shaft and to which at least one rotor blade is secured. The rotor blade comprises a reflector whose position changes in accordance with the loading of the rotor blade. A radiation source for electromagnetic radiation and a radiation receiver are arranged in the gondola, a radiation path being provided from the radiation source to the reflector and back therefrom to the radiation receiver. By means of a monitoring device, a redirection of the loaded rotor blade can be determined on the basis of a modification of the radiation received from the radiation receiver.
In the above-mentioned methods, oscillations of the wind turbine lead to a measurement error since these oscillations cause a movement of the camera. Consequently, the camera movement of a movement of the reflector image caused by a blade bending is superimposed on the photo-sensitive surface of the camera. Furthermore, a measurement error is caused by the fact that the camera is displaced owing to an irreversible deformation of the camera retention member and/or the rotor blade. An example of such a deformation is the change of an originally circular cross-section of the rotor blade close to the blade root into an oval cross-section, which can be attributed to material creep under load.
Conventional camera-based systems for measuring the blade bending at one or more locations along the rotor blade use a light source which illuminates one or more reflectors, which are arranged at predetermined positions along the blade. The light reflected by the reflectors is received by means of a digital camera which comprises a photo-sensitive surface for this purpose. By evaluating the position of the reflector images on the photo-sensitive surface, the displacement of the reflectors relative to the camera position is calculated and the blade bending is derived therefrom. In this instance, in order to maximise the measurement resolution in the camera, a lens is used with such a narrow viewing angle that a maximum displacement of the reflector image on the photo-sensitive surface is achieved when the reflector (or the reflectors) experience(s) a maximum displacement to be anticipated under blade loading. Therefore, the region of the blade root of the rotor blade is not generally located in the field of vision of the camera.
The following problems are in particular connected with conventional camera-based systems for measuring blade bending:                Owing to oscillations of the camera, the reflector image moves on the photo-sensitive surface of the camera so that the system falsely interprets and signals this movement as a bending action of the rotor blade.        The position of the camera relative to the blade root changes in an irreversible manner with increasing time, for example, owing to a deformation of the rotor blade in the region of the camera position owing to material creep (for example, oval cross-section of the blade) so that a displacement of the reflector image on the photo-sensitive surface is brought about even without blade bending. Consequently, there is produced an error in the measurement of the blade bending.        